This invention relates to digital radio receivers which are used for navigation systems and other ranging applications, wherein the received signals are encoded with a pseudo-random noise (PRN) type code. This invention deals specially with environments where the multipath fading is severe.
The United States government has placed into orbit a number of satellites as part of a global positioning system (GPS). A GPS receiver receives signals from several such satellites and can determine very accurate parameters, such as position, velocity, and time. There are both military and commercial uses. A primary military use is for a receiver in an aircraft or ship to constantly determine the position and velocity of the plane or ship. An example of a commercial use includes surveying and the accurate determination of the location of a fixed point or a distance between two fixed points, with a high degree of accuracy. Another example is the generation of a high accuracy timing reference.
In order to accomplish this, each satellite continually transmits two L-band signals. A receiver simultaneously detects the signals from several satellites and processes them to extract information from the signals in order to calculate the desired parameters, such as position, velocity or time. The United States government has adopted standards for these satellite transmissions so that others may utilize the satellite signals by building receivers for specific purposes. The satellite transmission standards are set forth in detail by an "Interface Control Document" of Rockwell International Corporation, entitled "Navstar GPS Space Segment/Navigation User Interfaces", dated Sep. 26, 1984, as revised Dec. 19, 1986.
Briefly, each satellite transmits an L1 signal on a 1575.42 Mhz carrier, usually expressed as 1540 f.sub.0, where f.sub.0 =1.023 Mhz. A second, L2 signal transmitted by each satellite, has a carrier frequency of 1227.6 Mhz, or 1200 f.sub.0. Each of these signals is modulated in the satellite by at least one pseudo-random signal function that is unique to that satellite. This results in developing a spread spectrum signal that resists radio frequency noise or intentional jamming. It also allows the L-band signals from a number of satellites to be individually identified and separated in a receiver. One such pseudo-random function is a precision code ("P-code") that modulates both of the L1 and L2 carriers in the satellite. The P-code has a 10.23 Mhz clock rate and thus causes the L1 and L2 signals to have a 20.46 Mhz bandwidth. The length of the code is seven days; that is, the P-code pattern is begun again every seven days. In addition, the L1 signal of each satellite is modulated by a second pseudo-random function, or a unique clear acquisition code ("C/A code"), having a 1.023 Mhz clock rate and repeating its pattern every one millisecond, thus containing 1023 bits. Further, the L1 carrier is also modulated by a 50 bit-per-second navigational data stream that provides certain information of satellite identification, status and the like.
In a receiver, in the process of demodulating those satellite signals, signals corresponding to the known pseudo-random functions are generated and aligned in phase with those modulated onto the satellite signals. The phase of the carriers from each satellite being tracked is measured from the results of correlating each satellite signal with a locally generated pseudo-random function. The relative phase of carrier signals from a number of satellites is a measurement that is used by a receiver to calculate the desired end quantities of distance, velocity, time, etc. Since the P-code encrypted functions (Y-code) are to be classified by the United States government so that they can be used for military purposes only, commercial users of the GPS must work directly only with the C/A code pseudo-random function.
The government of the former USSR has placed into orbit a similar satellite positioning system called GLONASS; more information on its standard can be found in the "Global Satellite Navigation System GLONASS-Interface Control Document" of the RTCA Paper No. 518-91/SC159-317, approved by the Glavkosmos Institute of Space Device Engineering, the official former USSR GLONASS responsible organization. Although the present invention is described herein for use with the United States GPS system, it can be applied to a receiver designed to acquire the GLONASS signals or any radio frequency system using pseudo-random noise sequences for ranging.
One of the major factors influencing the final accuracy of a distance, velocity, etc., measurement being made is the accuracy with which the signal phase measurements are made. In turn, this phase measurement precision is altered if, in addition to the direct line-of-sight propagation signal, a multipath fading signal is also received. The phase of the C/A code, for example, is determined by use of a delay locked loop (DLL) correlator, wherein the phase of the internally generated C/A PRN code sequence is adjusted in a control loop to minimize an error signal. The DLL uses early and late versions of the internally generated code in a signal correlator that is part of it. Many such receivers use a time spacing between the early and late versions of one PRN code chip. (A "chip" is the time during which the code remains at a plus or minus one.) Operation of the DLL within such receivers is affected by any multipath signal present, thus causing a tracking error. The phase locked condition of the DLL is not only controlled by the line-of-sight signal, as is desired in order to eliminate a cause of phase measurement errors, but is also affected by the multipath signals as well.
Errors caused by multipath distortion in the out-of-phase condition can be reduced by narrowing the delay spacing between the early and late correlators in the DLL. Although this technique reduces the effect of the received multipath signals somewhat by reducing the loop gain to the weaker multipath signals, inaccuracies still result. It has not been found to be possible to completely eliminate the tracking error by simply narrowing the early-late delay spacing.
The Prior Application identified above describes use of a correlator having a zero loop gain over a majority of a range of relative phase difference between the locally generated PRN code and that encoded in the radio frequency signal being received from a satellite or other source, while, at the same time, providing a finite magnitude of loop gain in an operating phase difference region positioned about a zero phase difference. This significantly reduces, and even, in some cases, eliminates phase measurement errors that result when one or more multipath versions of a PRN encoded signal are present. However, as the desired level of measurement precision has increased, a phase measurement accuracy even greater than that provided by the techniques described in the Prior Application has been sought.
Therefore, it is a primary and general object of the present invention to provide a receiver signal processing technique that substantially eliminates phase measurement errors resulting from the presence of one or more multipath signals being received along with the desired line-of-sight signal.